    Patent Document 1: Japanese Laid-Open Patent Application No. 2001-38564;    Patent Document 2: Japanese Laid-Open Patent Application No. H08-155770;    Patent Document 3: Japanese Laid-Open Patent Application No. 2003-39264.
In general, a machining tool, such as a machining center, has a machining table. A work pallet is positioned and clamped to the machining table, and a work piece clamped on the work pallet is machined. Some of precision machinery components require a machining accuracy of approximately 1 μm. An error in positioning the work pallet relative to the table affects the machining accuracy of the work piece. In other words, the positioning accuracy of the pallet relative to the table is extremely important. Various devices for positioning and clamping the pallet to a base member fixed to the table, or to the table itself as the base, have been proposed and put into practical use.
A clamping device having a datum function disclosed in Japanese Laid-Open Patent Application No. 2001-38564 (Patent Document 1) has a cylindrical shaft projecting from the base. A tapered collet is fitted slidably onto the cylindrical shaft, and the tapered collet is pressed elastically upward by a spring. The work pallet has an annular bushing capable of being fitted onto the cylindrical shaft. With the bushing fitted onto the cylindrical shaft, the pallet is pulled down and clamped to the base by a clamping mechanism. The tapered collet is fitted tightly in between the cylindrical shaft and an annular tapered surface of the bushing, whereby the pallet is positioned in the horizontal direction relative to the base. Further, a horizontal surface of the bushing abuts a receiving surface of the base from above, whereby the work pallet is positioned in the vertical direction in relation to the base.
A seating device for the pallet disclosed in Japanese Laid-Open Patent Application No. H08-155770 (Patent Document 2) have a tapered cone projecting from a base member. The pallet has a tapered socket capable of being fitted onto the tapered cone, and an annular groove for facilitating elastic deformation is formed on the inner periphery of the tapered cone. With the tapered socket being fitted onto the tapered cone, the pallet is pulled down and clamped to the base by a clamping mechanism. The tapered socket is engaged tightly with the tapered cone while the tapered cone is subject to slight elastic deformation for radial contraction and pressed tightly against the annular tapered surface of the tapered socket, by which the pallet is positioned in the horizontal direction relative to the base. Further, a horizontal surface of the tapered socket abuts a receiving surface of the tapered cone from above, whereby the pallet is positioned in the vertical direction in relation to the base.
A work pallet positioning and clamping device disclosed in Japanese Laid-Open Patent Application No. 2003-39264 (Patent Document 3) has a cylindrical shaft projecting from the base. The pallet has an annular bushing capable of being fitted onto the cylindrical shaft. The bushing has a nearly annular engaging mechanism that is elastically deformable in the radial direction, on the inner periphery, and an annular escape groove, for allowing for the elastic deformation of the engaging portion, on the outer periphery. Further, multiple slits are formed on the annular engaging mechanism. With the bushing being fitted on the cylindrical shaft, the pallet is pulled down and clamped to the base by a clamping mechanism. The engaging portion of the bushing is engaged with the cylindrical shaft, while the engaging mechanism of the bushing is elastically deformed outward, radially, toward the annular escape groove, and pressed against on annular tapered surface of the cylindrical shaft, whereby the pallet is positioned in the horizontal direction in relation to the base. Further, a horizontal surface of the bushing abuts a receiving surface of the base from above, whereby the pallet is positioned in the vertical direction relative to the base.
In the clamping mechanisms of Patent Documents 1 and 3, a power output member is introduced in the axial bore of the cylindrical shaft. Multiple, radially movable steel balls are held radial holes formed in the top part of the cylindrical shaft. When the power output member is pulled down, the multiple steel balls are pushed outward, radially, by a slanted surface on top part of the power output member. The steel balls are engaged with a annular engaging surface formed in the bushing, whereby a clamping force for pulling down and clamping the pallet to the base is produced. Several steel balls (for example, six) are used, so that a significantly large force is exerted on the annular engaging surface when the steel balls are engaged.
However, the devices of Patent Documents 1 to 3 have the following problems:
In the clamping device having a datum function of Patent Document 1, the tapered collet and the cylindrical shaft are formed separately. The tapered collet is fitted slidably onto the cylindrical shaft, pressed elastically upward by a spring, and stopped by a stopper ring. The structure having such a tapered collet increases the number of parts, thereby increasing production costs. Repeated use of the clamping device causes the cylindrical shaft and tapered collet to wear out at the sliding part, and, further, accumulated abrasion powder may significantly reduce positioning accuracy. In addition, there is a small gap between the cylindrical shaft and the tapered collet, thus machining oil and cutting powder entering that gap may significantly reduce positioning accuracy.
In the seating device, for the work pallet of Patent Document 2, the tapered cone is elastically deformed for radial contraction. The tapered cone has a relatively thick, annular continuous wall. Therefore, it is difficult for the tapered cone to be elastically deformed for radial contraction. In other words, its radial contraction is so small that the tapered cone and tapered socket has to be produced with high production accuracy in order for the surface of the tapered socket to abut the receiving tapered surface of the tapered cone, which makes the entire device expensive. By increasing the force of the clamping mechanism, the tapered cone can be subject to large elastic deformation for radial contraction. However, upon receiving such a significant force, the annular tapered surface of the tapered cone tends to partially wear out, failing to ensure prolonged positioning accuracy. Further, when the tapered cone is subject to large elastic deformation for radial contraction, the elastic part experiences excessive stress. The elastically deformed part may not be restored to its proper shape, and the deformation may become permanent.
In the positioning and locking device disclosed in Patent Document 3, the annular engaging mechanism of the bushing has, on its outer periphery, an annular escape groove that allows for the elastic deformation of the engaging mechanism, and, further, multiple slits, so that the engaging mechanism is easily elastically deformed outward, radially. However, if the engaging mechanism is engaged with the cylindrical shaft for pulling down and clamping the pallet to the base before the central axes of the cylindrical shaft and bushing are not aligned (for example with a slight misalignment), the engaging mechanism is, as a result of the engagement, subject to localized elastic deformation, and pressed against the annular tapered surface without the central axes of the cylindrical shaft and bushing being aligned. Thus, in this state, the pallet may be clamped to the base. In other words, when the pallet carries a heavy load, the pallet may not be reliably or highly accurately positioned in the horizontal direction relative to the base.
Further, in the clamping mechanisms in Patent Documents 1 and 3, when the annular engaging surface of the bushing have an annular tapered surface, with which multiple steel balls are engaged for clamping, the steel balls make contact at a point with the annular tapered surface. This small contact area causes the annular tapered surface to receive a significant localized force. Therefore, particularly when a significant clamping force is applied, the steel balls may form trace of point contact on the annular tapered surface, subsequently, receives an uneven clamping force as the pallet is used repeatedly. It may therefore become difficult to maintain stable clamping. In such cases, the steel balls can be increased in size or number, however, such attempts result in enlarging the clamping device and are disadvantageous in terms of production cost.
There is also a demand for increased positioning accuracy in positioning and clamping a metal mold to a base, such as a movable or fixed board of a press machine or injection molding machine. When the devices of Patent Documents 1 to 3 are used, the same problems, as described above, occur.
The object of the present invention is to provide a positioning and clamping device that improves positioning accuracy in positioning a clamping object relative to a base member, decreases the number of components for reduced cost, and maintains high positioning accuracy and stable clamping after prolonged and repeated use, particularly by providing an annular engaging mechanism, which is elastically deformable in the radial direction, to a cylindrical shaft of the base member, or an annular engagement member of the clamping object, with the engaging mechanism having part and multiple ribs.